The present invention relates to a process for the manufacture of melamine cyanurate, in an aqueous medium, from cyanuric acid and melamine and in the presence of a strong mineral acid.
Several methods of synthesis have been proposed in the past for manufacturing melamine cyanurate.
Thus, it is known to manufacture melamine cyanurate by the reaction of cyanuric acid and melamine in an aqueous medium. According to Patent JP 56032470, the reaction is performed at a pH above 7, instead of pH 3 to 12 according to Patent PL 100877.
The synthesis in an aqueous medium may be carried out in the cold state (Patent JP 54055588-A): it is then very slow. A more important prior art relates to processes performed in the heated state. This is the case with Patent JP 55147266-A, which recommends, at the end of the reaction, a removal of water by concentration under vacuum. Similarly, Patent JP 54141792-A relates to a synthesis process at 90.degree. C., according to which the final product is recovered by filtration followed by drying at 105.degree. C.
Moreover, Patent JP 5405587 discloses a process for the manufacture of melamine cyanurate by the reaction of cyanuric acid and melamine in the presence of water in a kneader for 4 hours at 90.degree. C.
Lastly, Patent JP 54125690-A relates to a process for the synthesis of melamine cyanurate by mixing cyanuric acid and melamine in the solid state, followed by heating at 200.degree. C. The latter technique has the drawback of necessitating reactors equipped with powerful means of kneading.
The syntheses in an aqueous medium known hitherto are performed at a pH above 3, and necessitate large amounts of water. In effect, an insufficient dilution leads to the production of pastes, making the use of kneaders obligatory. The removal of such amounts of water at the end of the reaction can become a crippling limitation of these processes, inter alia on account of the excessive time it takes.
Furthermore, the melamine cyanurate obtained by these processes in an aqueous medium has a very fine particle size, so that its filtration by simple means leads to a clogging of the filters unless more complicated filtration techniques, such as tangential filtration, are used. This final product is, in addition, very difficult to wash, and contains impurities, chiefly consisting of unreacted cyanuric acid and melamine.